This invention relates to a method of producing a thin oriented silicon steel sheet and to the decarburized steel sheet for a thin oriented silicon steel sheet product having a forsterite coat of reduced thickness which is uniform and improved in adhesion, and which has good magnetic characteristics.
As magnetic characteristics of an oriented silicon steel sheet, a high magnetic flux density and a small core loss are required.
After the recent energy crisis, trials have been made to reduce the energy loss of transformers, generators and the like. With this movement, needs for low-core-loss material for oriented silicon steel sheets have been increased. For reducing core loss, reducing the thickness of each steel sheet so that its electrical resistance is increased is most effective. Various studies have therefore been made to enable production of thinner steel sheets by gradually reducing the sheet thickness from about 0.30 mm to 0.28, 0.23, 0.20 and 0.18 mm.
With the reduction in thickness, oriented silicon steel sheets have actually been improved in core loss. However, a problem has then arisen in that when transformers are actually manufactured by using such silicon steel sheets, the energy loss reduction effect is not significantly large, contrary to expectation.
This is because as the thickness of steel sheets is reduced and the thinner sheets are used in a laminated arrangement when a transformer is assembled, the proportion of the volume occupied by the iron portions to the total volume of the core (hereinafter referred to as the "space factor") becomes smaller. The reduction of the space factor is mainly due to an increase in the proportion of the tensile coating layer and the forsterite coat formed under this layer.
Accordingly, if the thicknesses of these coating layers could be sufficiently reduced while the thickness of the steel sheet is also reduced, the space factor of the iron portions of the laminated structure might even be increased, in which case the problem would be solved. However, it is, in fact, difficult to reduce the coat thickness as well as the sheet thickness for the following reason. The thickness of the tensile coating can be reduced comparatively easily because the tensile force to be applied is reduced in proportion to the reduction of the steel sheet thickness. However, if the thickness of the forsterite coat is reduced, various surface coating characteristics, such as insulation performance, rust proofing performance, uniformity and adhesion, deteriorate simultaneously.
The forsterite coat is formed mainly by a solid phase reaction which takes place during finishing annealing. The reaction takes place between silica (SiO.sub.2) in a subscale formed as an outer layer of the steel sheet during decarburization/primary-recrystallization annealing and magnesia (MgO) in an annealing separator applied to the steel sheet surface. This reaction is basically EQU 2MgO+SiO.sub.2 .fwdarw.Mg.sub.2 SiO.sub.4.
Accordingly, to reduce the thickness of the forsterite coat it is necessary to reduce the amount of silica in the subscale formed by decarburization/primary-recrystallization annealing. However, it is known that if the amount of silica in the subscale is reduced the uniformity of forsterite coat formation is impaired and the adhesion and uniformity of the coat deteriorate. In conventional processes, therefore, the amount of oxides in the subscale formed during decarburization/primary-recrystallization annealing is controlled so as to be constant irrespective of the product sheet thickness. This is described in Japanese Laid-Open Patent Publication No.56-72178 or Japanese Patent Publication No.62-53577. For example, according to Japanese Patent Publication No.62-53577, the amount of oxygen per unit area (hereinafter referred to as the "marked oxygen" amount, which is generally proportional to the thickness of the forsterite coat) calculated is within the range of 0.7 to 1.4 g/m.sup.2 irrespective of the sheet thickness, and is controlled to be generally constant. To form such a desirable coat, the marked oxygen amount in the step of decarburization/primary-recrystallization annealing is set to a constant value irrespective of the product sheet thickness, so that the thickness of the forsterite coat is constant. It is therefore difficult to form a forsterite coat on a thinner steel sheet while reducing the thickness of the forsterite coat as well as the overall thickness of the steel sheet. With a reduction in the steel sheet thickness, the problem of deterioration of magnetic characteristics also arises.
Generally, it is necessary to sufficiently grow secondary-recrystallized grains having an orientation called Goss orientation in the (110)[001] direction during finishing annealing in order to obtain an oriented silicon steel sheet having good magnetic characteristics.
Secondary-recrystallized grains having Goss orientation grow by nucleus generation in the vicinity of an outer layer of the steel sheet. For suitable secondary recrystallization it is necessary effectively to inhibit the normal growth of primary grains of other orientations by a precipitate called an inhibitor. However, the inhibitor in the outer layer of the steel sheet is easy to oxidize in a weakly oxidizing atmosphere during finishing annealing, so that the inhibition effect in the outer layer of the steel sheet is necessarily lost during finishing annealing. The nucleation frequency of secondary-recrystallized grains per unit surface area is reduced according to the reduction in the sheet thickness, and the nucleus generation positions become closer to the steel sheet surface with the reduction in the sheet thickness. Nucleation regions are therefore formed closer to the outer layer in which the inhibition effect of the inhibitor is lost, so that it is difficult to promote secondary recrystallization. There is therefore a critical sheet thickness.
The subscale formed at the steel sheet surface generally inhibits oxidation of the outer layer of the steel sheet, i.e., it protects against the weakly oxidizing atmosphere and therefore serves to prevent a reduction in the outer layer inhibition effect. However, if the coating thickness is reduced, the marked oxygen content of the subscale and hence the thickness of the subscale are reduced, which makes it further difficult to promote secondary recrystallization.
It is known that addition of Sb to the steel material is effective against such oxidation. This addition is intended to limit the oxidation effect of the atmosphere by utilizing segregation of Sb to the steel sheet surface, and has a significant oxidization limiting effect. However, addition of Sb simultaneously reduces the effect of the subscale in protecting the inhibitor against the atmosphere during finishing annealing, because Sb acts to deteriorate important properties of the subscale. This means is therefore commercially unsatisfactory.
Because decarburization/primary-recrystallization annealing has significant effects as described above, various atmosphere/temperature patterns for this annealing have been studied. However, they have been proposed to realize improvements in coating characteristics and magnetic characteristics and are necessarily intended to set a certain marked oxygen amount such that a thick coat is formed.
For example, Japanese Patent Publication No.57-1575 discloses a method of separating a decarburization/primary-recrystallization annealing step into first and second steps and reducing the oxygen potential P(H.sub.2 O)/P(H.sub.2) in the second step relative to that in the first step. Japanese Patent Publication no.54-24686 discloses a method of effecting decarburization/primary-recrystallization annealing at a temperature of 750.degree. to 870.degree. C. and thereafter effecting annealing in a non-oxidizing atmosphere at a high temperature of 890.degree. to 1,050.degree. C. before finishing annealing.
These methods, however, are intended to maintain a certain marked oxygen amount for sufficient decarburization, that is, to improve magnetic/coating characteristics by forming a thick subscale and do not enable formation of a thin coat.
Techniques intended to reduce the forsterite coat are disclosed in Japanese Patent Publication Nos. 58-55211 and 62-53577, but they are not based on studies of decarburization/primary-recrystallization annealing with respect to technical means for improving the coating characteristics while reducing the coat thickness, and are therefore unsatisfactory in terms of industrial production.